The proposed study is concerned with development and validation of indirect tests (frequency dependence of resistance and two gases of different densities--two gas technique) for the detection of mechanical abnormalities specific to the small airways of the lung. The study is proposed to proceed in three stages: 1) development of the two indirect tests in question, that is, frequency dependence of resistance and two gas technique for use in humans, dogs and ponies and establish the direct peripheral airway catheter technique in the animal models for partitioning the total airway resistance into its central and peripheral components. 2) Comprehensive evaluation of these indirect techniques with the aim of assessing their ability to meet specific criteria which we feel are requisite to a technique that indirectly measures small airway function. This stage has two steps: a) assessment of these tests' reproductibility, sensitivity, precision and practicality; b) validity and specificity of the test will be determined exclusively in the animal model using a measurement of peripheral resistance as a criterion technique. These procedures will determine the indirect tests' accuracy to measure perturbation in peripheral airway resistance and the limitations to their specificity and sensitivity. 3) Finally, the indirect technique(s) which, from the animal studies, has been shown to best meet our established criteria will be applied to specific patient populations.